Piezoelectric quartz crystals used as bandpass filters provide a simple means for obtaining high selectivity in the i.f. sections of radio receivers primarily because of their extremely high Q ratings. Such filters generally include a resonator structure disposed on surface of the quartz. The high Q rating of such filters practically eliminate any audio frequency imaging problems while also providing good discrimination against adjacent signals and reducing broadband noise. In addition to the operational advantages and ease of use of crystal filters, they are also economically attractive.
In spite of the many advantages of crystal filters, their use in certain radio applications has been resisted because of the failure modes experienced by such filters. Specifically, monolithic crystal filters are prone to three types of failure modes:
A. Failure of input or output impedance matching networks PA1 B. Failure of a resonator PA1 C. Failure of the ground connection to a resonator.
Although failures in modes A or B result in an increased insertion loss and a moderate bandpass ripple, such failures are not catastrophic so long as the signal strengths are high enough to provide coupling through the failure. More particularly, if such filters are used in an aircraft instrument landing system receiver and fail in modes A or B, the minimum resulting distortion of the received signal permits its use in the instrument landing system in high signal areas with full confidence. However, failure of such filters in mode C result in severe bandpass ripple, causing distortion of the signals applied to the filters and altering the signal modulation sidebands. Failure mode C is particularly serious in instrument landing systems since the increased signal distortion caused by the loss of the resonator ground connection can cause the received signal to fall into erroneous areas of the passband to thus alter the difference in the depth of signal modulation. As known to those skilled in the instrument landing system arts, the difference in the depth of signal modulation is a measure of the aircraft deviation from the approach glide path, thus any alteration of the difference in the depth of signal modulation caused by failure in the circuitry of the instrument landing system receiver can result in dangerously erroneous aircraft position information being displayed to the pilot.
It has been suggested that mode C failures be prevented by a double grounding technique wherein the filter resonator is grounded through two separate and independent paths in the belief that it is highly unlikely that both grounds would fail before repairs could be effected. However, because of the small size of such filters and the manner in which they are packaged in protective enclosures, there is no convenient way to determine whether one of the ground connections has failed without disassembling the filter. Thus, this method of mode C failure protection is impractical.